Modern communication system providers are striving to increase the capacity of their systems to satisfy the rapidly growing exchange of information around the world. Increasing the data rate of a single wavelength channel is one strategy to increase the throughput on optical fibers. However, this approach is limited in that the data rate for a single optical channel will eventually reach practical limitations. An important strategy to further increase the available bandwidth is to add multiple wavelength channels. Multiple wavelength systems are referred to as being wavelength division multiplexed (WDM).
Optical communications systems are available with single-channel data rates at 10 Gbit/s and faster. To accommodate the spectral bandwidth of these signals, the channels in a WDM system are commonly spaced at 100 GHz, or ˜0.8 nm in the 1550 nm wavelength range. A device would have to be useful over bandwidths greater than ˜0.8 nm to be a truly multi-channel device in these WDM systems. Ideally a device would operate over a full communications band of wavelengths, so systems could be designed for any WDM or modulation scheme without needing to accommodate a specific dispersion correction module. Current communications bands are defined by optical amplifier operating ranges; for instance, the “C” band covers ˜1530 nm to ˜1560 nm and the “L” band covers ˜1570 nm to ˜1610 nm.
In these optical communications systems, short pulses of optical energy are sent through optical fibers to transmit information. These optical data pulses are comprised of a spectrum of wavelengths. Generally speaking, an unchirped pulse of duration t has a spectral width of ˜1/t, e.g., a ˜1 nanosecond (10−9 second) pulse has a ˜1 GHz (109 Hz) spectral width. As a pulse travels along standard singlemode fiber in the ˜1550 nm range, the shorter wavelength components travel faster than the longer-wavelength components. This effect, called chromatic dispersion, broadens the pulse to the point that it eventually interferes with neighboring pulses in a pulse train and introduces errors in the detected data stream. A number of solutions have been proposed for this problem, but only dispersion-compensating fiber (DCF) and chirped fiber Bragg gratings have been considered seriously as potential candidates for deployment.
Dispersion-compensating fiber has high levels of dispersion of opposite sign to that of standard fiber. To compensate for the dispersion introduced by an 80-km span of standard fiber, one would have to concatenate a ˜16-km length of DCF into the system. These compensation modules are bulky, and due to the fiber design, suffer high optical attenuation and increased optical nonlinear effects. However, DCF is used today since no serious alternative exists.
Fiber Bragg Gratings (FBGs) provide a promising solution for dispersion compensation. A fiber Bragg grating comprises an optical fiber or other optical waveguide having periodic, aperiodic or pseudo-periodic variations of the refractive index along its length in the light guiding region of the waveguide. Gratings are usually written in photosensitive optical fibers. The refractive index of photosensitive glass optical fibers may be changed by actinic radiation- that induces localized alterations of the glass structure. The term “actinic radiation” includes visible light, UV, IR radiation and other forms of radiation that induce refractive index changes in the glass. Introduction of changes in refractive index occurs by exposure of a photosensitive glass fiber to an interferogram of UV radiation. The period of the resulting fiber Bragg grating in the fiber corresponds to the period of the interferogram scaled by the refractive index of the waveguide.
To function as a dispersion compensator, the grating period of an FBG is chirped to reflect lagging wavelengths before faster wavelengths, which must travel further into the grating before they are reflected. An optical circulator separates the input of a dispersion compensation module (DCM) from the output. A dispersion compensating grating (DCG) module recompresses a data pulse that had been corrupted by chromatic dispersion and as a result, the optical system performance is enhanced. The longer the grating, the greater the DCG compression factor and the wider the bandwidth of the device.
Long length gratings for dispersion compensation are not readily available, since extreme tolerances must be maintained to manufacture quality long length gratings. Fabrication errors in chirped gratings create unwanted variations in the group delay curve, called group delay ripple, and thus inaccuracies in the dispersion correction. The impact of these ripples on optical system performance is poorly understood, but some system designers have predicted the need for these ripples to be less than ˜40 ps peak-to-peak for a DCG to be useful as dispersion compensator in most optical transmission systems. However, the magnitude of the ripple needed to make a useful FBG dispersion compensation device has not been verified. A ripple amplitude of ˜40 ps peak-to-peak can be caused by a 20% variation in the FBG UV-induced index change, a ˜0.3% dimensional change in a fiber core, or a ˜4 pm error in grating pitch. Since the inter-atomic spacing between silicon and oxygen atoms in glass is ˜160 pm, holding required tolerances during grating inscription is viewed as highly unlikely. This view identifies fiber fabrication tolerances as the limiting factor to the production of quality wide-band dispersion compensation gratings.
In 1995, a Swedish research group reported the fabrication of a long-length FBG by “stitching” smaller FBGs together. A small grating was written, the fiber was translated by a grating period through a UV-interferogram with a high-precision linear stage, and then the fiber was irradiated again. This process was continued until gratings of extended length, up to 50 cm were made. The range of motion of available high-precision staging limited the length of these FBGs. Other reports alleged manufacture of long length dispersion compensation gratings included irradiation of up to 2.5 meters of an optical fiber through a number of phase masks aligned sequentially along the length of the fiber. A similar process involved the use of a phase mask formed in the periphery of a disk. During rotation of the disk, ultraviolet radiation passing through the phase mask was expected to produce refractive index changes in an optical fiber as the rotating disk traversed a length of the optical fiber. Difficulties of rotational registration to produce a continuous length of phase mask apparently made this method impractical.
Stitching methods require precise knowledge of optical fiber location relative to the interferogram used to produce a long length FBG, otherwise group delay ripple characteristics are too large for effective dispersion compensation in optical communication systems. Precise positioning between a phase mask and an optical fiber depends upon the accuracy of the motion stage encoder, which suffers from interpolator inaccuracies, noise in edge detection electronic circuitry and random fluctuations in received interpolator laser light. Several feasibility studies have been completed where long-length FBGs, fabricated by stitching, have been used successfully at specific wavelengths as dispersion compensators in optical communication systems. Since the FBG delay ripple imposed very large distortion-derived system penalties at most wavelengths, the wavelength of the transmitting laser in the communication system had to be adjusted in these studies to obtain reasonable system performance.
A common procedure for determining chromatic dispersion of a device is the modulation-phase shift method, as described in Chapter 12 of Fiber Optic Test and Measurement (ed. D. Derickson, Prentice Hall PTR, N.J., 1998, ISBN #0-13-534330-5). The output of a narrowband, tunable optical source is intensity modulated and applied to the device under test. The transmitted (or reflected) signal is detected and the phase of its modulation is measured relative to the electrical modulation source. The phase measurement is repeated at intervals across the wavelength range of interest. The curve of the relative group delay is constructed by accumulating these group delay changes across the measurement wavelength range.
A common procedure for determining system power penalty of a device is described in Chapter 8 of Fiber Optic Test and Measurement (ed. D. Derickson, Prentice Hall PTR, N.J., 1998, ISBN #0-13-534330-5). The system power penalty is the difference in detector power level needed to maintain a given bit-error-ratio (BER) before and after placing an optical device in an optical transmission link. Signal distortion attributable to the structure of the dispersion compensating fiber grating accounts for this power difference after subtraction of optical insertion loss differences.
The group delay ripple is determined by fitting with least squares minimization a line or a low-order polynomial to the relative group delay curve, and then subtracting the polynomial from the curve. The remainder of the subtraction is the delay ripple. Typically this ripple is considered as being “high frequency” ripple, i.e. ripple with a periodicity of less than the channel bandwidth of a communications system, and “low frequency” ripple, i.e. ripple with a periodicity greater than that of a channel bandwidth. Herein high-frequency ripple will be considered as ripple with a periodicity of less than 80 pm (10 GHz at ˜1550 nm), and low-frequency ripple will be a ripple with a periodicity of greater than 80 pm. The high-frequency ripple adds an intra-pulse distortion to a communication signal that is difficult to correct, thus the high-frequency ripple is considered more critical than the low-frequency ripple, which merely adds a slight error to the dispersion correction.
Establishing a correlation between delay ripple amplitude and optical system performance has been confusing because different groups measure DCG characteristics in different manners and, often, are not explicit about their measurement procedures. The severity of optical phase fluctuations induced by a component is usually specified in terms of the group delay ripple, e.g. as a peak-to-peak value. However, the system performance degradation from a group delay ripple depends strongly on the period of the ripple. A characteristic period may not exist or may not easily be identified from a measured group delay response. The group delay fluctuations have two separate and independent effects. Part of the fluctuations that are at the order of the signal bandwidth affect the signal spectrum by imposing an average chromatic dispersion, whereas the remaining fluctuations, which contain higher order frequency variations, distort the signal electrical field in a similar way to spectral phase noise. The induced average dispersion is found as a linear fit to the group delay within the signal bandwidth. The remaining fluctuations cause a system penalty, which is proportional to the variance of the residual phase fluctuation within the signal bandwidth.
Phase ripple information may be extracted in the following manner. The group delay values, τexp, are measured in wavelength steps of Δλ, e.g. by the modulation phase shift method. The first step is to obtain the actual, undesired fluctuations, τfluct, that are to be analyzed. This is found by subtracting the ideal group delay, τideal, for which the chirped grating was designed, from the measured group delay:τfluct=τexp−τideal−τ0.The constant τ0 defines an arbitrary fixed group delay. The ideal group delay of the grating is calculated as             τ      ideal        ⁡          (      λ      )        =                    D        ⁡                  (                      λ            0                    )                    ⁢              (                  λ          -                      λ            0                          )              +                  S        2            ⁢                        (                      λ            -                          λ              0                                )                2            ,      where D(λ0) is the target dispersion at a reference wavelength, λ0, and S is the (constant) target dispersion slope (dD/dλ) of the dispersion compensation module (DCM). A sliding sinc-function window centered at wavelength λc and with a width of the signal bandwidth (˜20 GHz bandwidth for a 10 Gb/s signal), is applied to the group delay fluctuations, τfluct. At each position, the group delay fluctuations within this weighted window are evaluated to extract the residual dispersion and the standard deviation of the residual phase.
Although several studies have demonstrated that DCGs could be used as dispersion compensators for a single communication channel, useful wide bandwidth devices have not been demonstrated, mainly because the phase ripple amplitude of these devices was too large. Widely chirped DCGs with several WDM channels operating across their bandwidth have been demonstrated, but because the wavelength of the transmitting laser in the communication system needs adjustment in all of these studies to obtain reasonable performance, these DCGs proved to have only very narrow sections of usable bandwidth across their entire bandwidth. If a wideband DCG with a large phase ripple is useful only over a very narrow range as a dispersion compensator, then the utility of this widely chirped device is lost. Others have demonstrated DCGs in laboratory systems where DCGs have been used to correct the dispersion in several channels of a communication system, but a DCG that covers only part of a communication band has limited appeal, since the system must be specially engineered to accommodate such a device. Gratings with a bandwidth of at least a third of a communications band (˜10 nm) or half a band (˜15 nm) have some appeal, since fewer accommodations for the device must be made to use it in a communications system than narrower devices.
Although used widely today to solve chromatic dispersion problems in high-speed optical communications systems, new dispersion compensating fiber (DCF) designs have a requirement to match characteristics of dispersion and dispersion slope of opposite sign to those of a given transmission fiber. Typically, DCF designs don't exactly match the dispersion characteristics of their intended fiber and thus leave a residual dispersion that accumulates over multiple spans of transmission fiber. Since DCF designs can be complicated and difficult to manufacture, several transmission fibers do not yet have a matching DCF counterpart. In some cases, fabrication of a matching DCF appears unlikely for use across a wide bandwidth.
Table 1 includes commonly used optical transmission fibers, identified by type and supplier.
TABLE 1Optical Fibers for Telecommunication SystemsDispersionDispersion SlopeFiber TypeSupplier(ps/nm)(ps/nm2)κ (nm)SMF-28 ™Corning, Inc. (Corning, NY)170.058298LEAF ®Corning, Inc. (Corning, NY)4.20.08550Truewave ®-RSLucent (Holmdel, NJ)4.50.045100TeraLight ™Alcatel, Inc. (Nozay, France)80.057140
The net dispersion, Dnet, and the dispersion slope, Snet, of a transmission fiber span with a dispersion compensation module can be defined as:Dnet=Dtr+DDCSnet=Str+SDCwhere Dtr and Str are the dispersion and dispersion slope of the transmission fiber, and DDC and SDC are the dispersion and dispersion slope of the dispersion compensation module. To describe the amount of dispersion and dispersion slope requiring compensation, a parameter κ is commonly defined, which is the ratio of the dispersion D over the dispersion slope S, i.e., κ=D/S. To simultaneously compensate the dispersion and the dispersion slope of the transmission fiber, κtr=κDC at every wavelength. It is difficult to match the κ value at every wavelength due to the curvature of dispersion in both the transmission fibers and DCF-based dispersion compensation modules. The above table compares the values of dispersion, dispersion slope and dispersion/slope ratio of different types of fibers at 1550 nm. These fibers have κ values ranging from about 50 nm to about 300 nm, which means they need different amounts of dispersion and dispersion slope compensation.
There thus exists a need for wide bandwidth (i.e., greater than several WDM channel spacings and preferably a full communications band) dispersion compensation module, using a single chirped FBG that can compensate for the chromatic dispersion and dispersion-slope in lightwave communications systems across an entire bandwidth. There also exists a need in the art for a wide-bandwidth chirped fiber Bragg grating that has a low phase ripple amplitude. These and other needs are met by the present invention, as hereinafter described.